1. Field of the Invention
The present invention relates generally to switching power supplies and more particularly to switching power supplies of the floating regulator type.
2. Prior Art
A switching power supply is used to convert an unregulated direct current (d.c.) input into a regulated d.c. output. Modern power supplies of this type may include a power switch (transistor), a diode clamp, a filter and a regulator. The regulator may include an oscillator and a duty cycle controller and is used to control the power switch. The regulator typically is connected to the filter via a feedback path which helps keep the output d.c. voltage at a constant value regardless of variations in the load by sampling the voltage output and feeding the sampled voltage back into the regulator for duty cycle control. The duty cycle of the switching transistor determines the average d.c. voltage output with the duty cycle being proportional to the ratio of the output d.c. voltage to the input d.c. voltage. Switching is usually done at a constant frequency just above the audible range, although some regulators use a variable frequency with changing line and load. With some switching regulators, it is possible to change the switching frequency with an external capacitor. Higher frequencies in general are less efficient since transistor switching losses and ferrite-core losses increase. On the other hand, lower switching frequencies in the audible range may cause some components to "sing" and/or produce interference in certain external circuits powered by the regulator. A fast-recovery rectifier, or a Schottky barrier flyback diode, is usually used as a free-wheeling diode clamp to keep the switching transistor load line within safe operating parameters and to increase overall efficiency. The power switching transistor needs to have a breakdown voltage rating greater than the input voltage, since at start-up the transistor sees the entire d.c. input voltage.
Switching power supplies can provide regulated outputs that are greater or smaller than the input, as desired. When the output voltage is greater than the input voltage, the power supply also referred to as a d.c.-d.c. converter is called a step-up (or boost) converter. Alternatively, a step-down (or buck) converter has an input voltage greater than its output voltage. The conversion in this case may be from a high-voltage unregulated input to a low-voltage regulated output.
Switching regulators can also be classified as to how they control the d.c. output voltage. Two conventional approaches are pulse width modulation (PWM) and pulse frequency modulation (PFM). Both approaches control the output voltage by varying the duty cycle. The PWM regulator operates by chopping the d.c. input voltage into pulses whose amplitude is the magnitude of the input voltage and whose duty cycle is controlled by a control circuit in the regulator in response to a sampled output voltage feedback.
Conventional PWM switching power supplies exhibit a number of problems such as high RFI emissions, complex circuit design, inefficient operation and audible noise during operation. Various designs have been proposed to overcome some or all of the above problems, none successfully. For example, U.S. Pat. No. 4,030,024 to Chambers et al discloses a flyback type of preregulator power supply having circuitry which is floating alternately from the input and output leads to accomplish direct coupling to a power switching regulator. The output choke has an added series winding which is connected to the output of the switching regulator and forms the return lead for the floating drive and control circuits. The complex circuit includes two standard 555 timers, a control operational amplifier, two transistors, a potentiometer and other components. The added series winding on the output choke may contribute to an increase in acoustic noise during operation of the power supply. This setup employs complex circuitry which raises the cost of the power supply
In other known switching power supplies, the power switch is typically a P-channel device in which case a complex circuit is needed to level shift the output of the regulating circuit up or down depending on whether a step-up or step-down converter is needed. This adds to the overall cost of the device and results in inefficient operation. In these devices, the regulator is usually connected to circuit ground.
Therefore, the need arises for an improved PWM switching power supply which operates efficiently and has simplified circuit architecture which may include a greatly reduced number of inexpensive off-the-shelf electronic components such as an inexpensive 555 timer, an N-channel MOSFET, an output filter and a flyback Schottky diode. Such an improved switching power supply should also preferably employ a PWM regulator which floats with the switching transistor rather than being connected to circuit ground. Such a power supply should preferably be a "quiet" power supply, that is, it should be designed in such a way as to reduce the audible noise emitted by conventional switching power supplies.